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Chapter 5Force and Motion

We have seen that part of physics is a study of motion, including acceleration which are changes in velocities.

Physics also a study of what can cause an object to accelerate. That cause is a force.

Force : a push or a pull Force is a vector : it has magnitude and

direction. S.I Unit s : Newton (N)

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5-3 Newton’s First Law

5-2 Newtonian Mechanics The relation between a force and the acceleration it causes was

first understood by Isaac Newton (1642-1727). The study of that relation, as Newton presented it, is called

Newtonian mechanics.

Imagine sending a puck sliding over the ice of a skating rink. More the surface is slipper, the puck would slide farther and farther.

In the limit you can think of a long, extremely slippery surface (frictionless surface), we can conclude that a body will keep moving with constant velocity if no force acts on it.

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Newton’s First Law : If a body is at rest, it remains at rest. If it is moving, it continues to move with the same velocity.

5-4 Force Since forces are vector quantities, they combine according

to the vector rules of chapter 3. When two or more forces act on a body, we can find their

net force or resultant force by adding the individual forces vectorially.

Forces are most often represented with a vector symbol such as .

A net force is represented with the vector symbol .

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The more proper statement of Newton’s First Law in terms of a net force : If no net force acts on a body (), the body’s velocity cannot change.

5-5 Mass Everyday experience tells us that a given force produces different

magnitudes of acceleration for different bodies. Put a baseball and a bowling ball on the floor and give both the

same sharp kick. The baseball receives a noticeably larger acceleration than the bowling ball.

Why: Because the mass of the baseball differs from the mass of the bowling ball.

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The mass of the body is the characteristic of that body that relates the body’s acceleration to the net force causing the acceleration.

In other word the mass is a measure of how difficult it is to change its velocity.

5-6 Newton’s Second Law The net force on a body is equal to the product of the body’s

mass and its acceleration.

Which may be written in the component versions:

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5-7 Some Particular Forces Gravitational force : on a body is a pull by another body (In

most situations, the other body is earth).

: The force is directed down toward the ground. The weight, W: Is the magnitude of the upward force

needed to balance the gravitational force on the body.

Normal force, : Is the force on a body from a surface against which the body presses. is always perpendicular to the surface.

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Frictional force, : Is the force on a body when the body slides or attempts to slide along a surface.

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Tension, : When a cord is under tension, each end of the cord pulls on a body. The pull is directed along the cord, away from the point of attachment to the body.

5-8 Newton’s Third Law Two bodies are said to interact when they push or pull on

each other.

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Newton’s Third Law : When two bodies interact, the forces on

the bodies

from each other are always equal in magnitude and opposite

in direction.

For the book and crate, we can write this law as the scalar

relation

Or as the vector relation

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5-9 Applying Newton’s Laws

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